Musculoskeletal complications, such as osteoporosis and aging related osteopenia, are major societal and health problems. Load-induced intracortical bone fluid flow is proposed as a critical mediator in initiating and regulating bone surface and osteonal adaptation. Using oscillatory pressurized marrow fluid flow stimuli, the physiological fluid stimulus was found to initiate new bone formation and reduce intracortical bone porosities caused by disuse, even in the absence of direct tissue strain. The new bone formation and inhibition of resorption were found to correlate with quantified flow parameters, i.e., fluid pressure gradients. This flow initiated bone adaptation occurs at a specific frequency range, i.e., 20-30 Hz, and is interdependent with the dose of anabolic fluid pressure. While bone remodeling was demonstrated to be sensitive to high strain frequency and low intensity physiological loading, the role of fluid flow perhaps explains, at least in part, the cellular response mechanism to anabolic stimuli. In the work proposed, we will examine the general hypothesis that bone fluid flow, mediated at specific physiological magnitudes and high frequencies, promotes osteogenic adaptation. Indeed, improving our understanding in which mechanical signals influence the temporal and spatial dynamics of bone remodeling may help to devise a biomechanically based intervention for treating osteoporosis, accelerating fracture healing or promoting bony ingrowth into prostheses. In this revised application (1-R01-AR049286-01), the goal will be achieved by a series of sub-hypotheses and specific aims: (1) The role of anabolic fluid flow, driven by daily intramedullary pressure (IMP), can initiate surface adaptive response and inhibit intracortical bone loss in a disuse bone. The remodeling response will be evaluated in a disuse in-vivo model in the absence of matrix strain following 4-week exposure of a short period of daily stimuli, consisting of a series of frequencies (0.5,1,5,10,20 & 40 Hz). (2) Osteogenic response to anabolic fluid flow stimuli is fluid pressure sensitive associated with the rate/frequency of loading. The anabolic potential response to hydraulic intensity will be evaluated in a disuse model following 4-week of daily ImP at 10, 20 and 80 mmHg with 1,5, and 20 Hz. (3) The potentials of fluid flow initiated adaptation are interdependent with specific fluid components, i.e., pressure gradient and fluid shear stress, which are responsible for restoring or inhibiting bone loss and new surface bone formation. A poroelastic finite element analysis will be developed, which will evaluate the correlation between fluid flow and resultant adaptation. (4) The osteogenic potentials response to fluid flow stimuli is initiated by osteoblastic activation of bone lining cells, following a daily but short duration (e.g., <10 days) of loading. Ultrastructural osteoblastic features of cell and nuclei will be examined via histomorphometric analysis of cell area, nuclear area, cell number, cell and nuclei shapes, in which associated fluid components will be identified.